In computing, a northbridge (also host bridge, or memory controller hub) is a microchip that comprises the core logic chipset architecture on motherboards to handle high-performance tasks, especially for older personal computers. It is connected directly to a CPU via the front-side bus (FSB), and is usually used in conjunction with a slower southbridge to manage communication between the CPU and other parts of the motherboard.

Historically, separation of functions between CPU, northbridge, and southbridge chips was necessary due to the difficulty of integrating all components onto a single chip die. However, as CPU speeds increased over time, a bottleneck emerged due to limitations caused by data transmission between the CPU and its support chipset. The trend for integrated northbridges began near the end of the 2000s –for example, the Nvidia GeForce 320M GPU in the 2010 MacBook Air was a northbridge/southbridge/GPU combo chip.

On older Intel based PCs, the northbridge was also named external memory controller hub or graphics and memory controller hub if equipped with integrated graphics. Increasingly these functions became integrated into the CPU chip itself, beginning with memory and graphics controllers. Since the 2010s, die shrink and improved transistor density have allowed for increasing chipset integration, and the functions performed by northbridges are now often incorporated into other components such as southbridges or CPUs themselves.

Intel and AMD have both released chipsets in which all northbridge functions had been integrated into the CPU. The corresponding southbridge was renamed by Intel as the Platform Controller Hub and by AMD as the Fusion controller hub. AMD FX CPUs continued to require external northbridge and southbridge chips. Modern Intel Core processors have the northbridge integrated on the CPU die, where it is known as the uncore or system agent.

The northbridge typically handles communications among the CPU, in some cases RAM, and PCI Express (or AGP) video cards, and the southbridge. Some northbridges also contain integrated video controllers, also known as a Graphics and Memory Controller Hub in Intel systems. Because different processors and RAM require different signaling, a given northbridge will typically work with only one or two classes of CPUs and generally only one type of RAM.

There are a few chipsets that support two types of RAM; generally, these are made available when there is a shift to a new standard. For example, the northbridge from the 2002 Nvidia nForce2 chipset only worked with Socket A processors combined with DDR SDRAM.

The Intel i875 chipset will only work with systems using Pentium 4 processors or Celeron processors that have a clock speed greater than 1.3 GHz and utilize DDR SDRAM, and the Intel i915G chipset only works with the Intel Pentium 4 and the Celeron, but it can use DDR or DDR2 memory.^{[citation needed]}

Northbridge chips are most commonly found on X86-based PCs, but they can also be found on other platforms, such as the PowerPC platform. A common example of a northbridge on a PowerPC platform is in Apple's older PowerPC-based computers like the iMac G5, which utilized an IBM CPC945 Northbridge chip. According to an Apple Developer note, The Power Mac G5's northbridge chip connected to a "Mid Bridge", which then connected to a south bridge.